Avoid gcc warning.

Cwd.xs:200:50: warning: size argument in 'strlcat' call appears to be size of the source; expected the size of the destination [-Wstrlcpy-strlcat-size]

No effective difference since both the source and destination
are buf[MAXPATHLEN].  They are now.
POSIX math: There's no #elifdef, unfortunately.
POSIX math: Cygwin lacks at least nexttoward.
POSIX math: let's not override the real lrint().
POSIX math: Use rounding macros, instead of the c99_ functions.

This avoids the need for forward declarations.  Also rename
the functions with UPPERCASE so people are more careful.