[posix] E: Add libposix pathconf/fpathconf stubs

[esaurez]

Summary

Adds minimal extern "C" stubs for pathconf(2) and fpathconf(2) to libposix's src/libs/posix/src/dummy.rs. Both stubs return -1 with errno = ENOSYS, matching the convention already used by every other "not implemented" stub in that module (popen, wordexp, glob, ftw, realpath, ...).

Why this is required

libstdc++'s std::filesystem code emits a strong undefined reference to pathconf from at least two compilation units:

• libstdc++-v3/src/c++17/cow-fs_ops.cc — std::filesystem::current_path(std::error_code&)
• libstdc++-v3/src/c++17/fs_ops.cc — std::filesystem::current_path[abi:cxx11](std::error_code&)

std::filesystem::current_path() calls pathconf("/", _PC_PATH_MAX) to size the buffer it passes to getcwd(). Without a definition for pathconf anywhere in the link, any executable that links libstdc++ (CPython, any C++ user binary that touches std::filesystem) fails with undefined reference to 'pathconf'. Concretely, this blocks ./z build of nanvix/cpython:

i686-nanvix-ld: libstdc++.a(cow-fs_ops.o): in function `std::filesystem::current_path(std::error_code&)':
cow-fs_ops.cc:(.text.../current_pathERSt10error_code+0x21): undefined reference to `pathconf'
collect2: error: ld returned 1 exit status
configure: error: C compiler cannot create executables

There is no existing implementation of either function in libposix today (verified by grep -r 'fn pathconf' src/libs/posix).

What changed

Just src/libs/posix/src/dummy.rs: two new public extern "C" functions in the "Standalone Functions" block, with full doc comments (Description / Parameters / Returns / Notes / Safety) and the standard #[trace_libcall] attribute. No other files touched. 79 insertions, 0 deletions.

pub unsafe extern "C" fn pathconf(_path: *const c_char, _name: c_int) -> c_long {
    ::syslog::debug!("pathconf(): not implemented");
    *__errno_location() = ErrorCode::InvalidSysCall.get();
    -1
}

pub unsafe extern "C" fn fpathconf(_fd: c_int, _name: c_int) -> c_long {
    ::syslog::debug!("fpathconf(): not implemented");
    *__errno_location() = ErrorCode::InvalidSysCall.get();
    -1
}

Why this stub is sufficient for libstdc++ today

libstdc++'s std::filesystem code path is defensive: it checks pathconf(...) == -1 and falls back to a compile-time PATH_MAX default (getcwd is called with a fixed 1024-byte buffer that grows on ERANGE). It does not look at errno. So returning -1 with any errno value, including the ENOSYS we use here, is sufficient to unblock the link without any observable behaviour change vs. "the system advertises no path-conf limit".

Future work (not in this PR)

A real implementation should return concrete, selector-aware values for the well-defined _PC_* selectors and only set errno = EINVAL for genuinely unrecognised selectors, following the musl src/conf/pathconf.c pattern:

Selector	Suggested value	Source
_PC_PATH_MAX	4096	nanvix <limits.h>
_PC_NAME_MAX	255	nanvix <limits.h>
_PC_LINK_MAX	1 (no hard links today)	nanvix VFS state
_PC_PIPE_BUF	4096	nanvix pipe buffer size
_PC_NO_TRUNC	1	nanvix never truncates names silently
... (others)	-1 with errno unchanged per POSIX "no determinate limit"	—

For now the stub returns -1 with errno = ENOSYS for every selector. The doc comment calls this out as a known limitation and points to the future-work direction. Users that rely on real values can follow up separately.

Validation

• In-tree CI checks pass locally: .\z.ps1 build -- format-check rust-lint-check spellcheck is green; pre-commit hook passes.
• End-to-end: against a Nanvix tree with these stubs applied, CPython 3.12 links against libstdc++ (previously failed on pathconf strong UND), the resulting python.elf runs hello.py, and import numpy; numpy.array(...).sum() via dlopen() of numpy 1.26.4's extension .sos produces NUMPY_TEST_OK on the Nanvix microvm.

Compatibility

• No public API change beyond adding two new C ABI symbols.
• No change to behaviour for any existing caller (the symbols were not defined before).
• The ENOSYS errno mirrors every other "not implemented" stub in dummy.rs for consistency. A future PR can refine to selector-aware values as described above without breaking this contract.