A few more minor file errno-reporting bugs.

A few more minor file errno-reporting bugs.

* callproc.c (Fcall_process):
* doc.c (Fsnarf_documentation):
* fileio.c (Frename_file, Fadd_name_to_file, Fmake_symbolic_link):
* process.c (set_socket_option):
Don't let a constructor trash errno.
* doc.c: Include <errno.h>.