include cleanups: remove unused headers and add feature test macros

debloat code that depends on /proc/self/fd/%d with shared function

I intend to add more Linux workarounds that depend on using these
pathnames, and some of them will be in "syscall" functions that, from
an anti-bloat standpoint, should not depend on the whole snprintf
framework.

fix various bugs in path and error handling in execvp/fexecve

implement fexecve