embed.fnc: Add EPTRQ constraints

Generally, a pointer to a string upper bound actually is to one beyond
the actual final byte in the string.  This is sanctioned by the C
Standard, and allows you to just subtract the lower bound from it to get
its length, without having to add 1.

But some functions are written to tolerate the upper bound pointer being
set to the actual final byte.  The EPTRQ constraint in embed.fnc is used
for those; the assertion becomes 'l <= u' instead of strictly less-than.

This commit is the first to use this type of constraint, and it applies
it only to those functions whose documentation or behavior clearly
indicate this is expected.

I removed now redundant asserts that were in the functions, and now are
included in the ARGS_ASSERT macros

There's a dozen-ish ones where that isn't true.  And they need to be
investigated further before deciding their disposition.

Author: khwilliamson