Random minor fixes for movemail

Random minor fixes for movemail

* movemail.c: Include <stdbool.h> and <signal.h>.
(waitpid) [WINDOWSNT]: New macro.
(wait) [WINDOWSNT]: Remove.
(main, popmail, pop_retr, mbx_write, mbx_delimit_begin)
(mbx_delimit_end): Use bool for boolean.
(main): Simplify #if usage a bit.
(main): Don't assume EOF == -1.  Prefer 'return' to 'exit'.  Don't
possibly unlink lockname twice, as that's a race condition.  Set
SIGCHLD to SIG_DFL to work around SysV misfeature.  Check for fork
failure.  Use waitpid, not wait, to avoid a race condition in the
unlikely case where we start up with a child.
(NOTOK, OK): Remove, in favor of plain boolean.
(popmail, pop_retr): Don't get confused about errno, e.g., ferror
need not set errno.
(popmail): Use fclose (mbf), not close (fileno (mbf)), to also
detect any stream-related errors (e.g., memory exhaustion).
(pop_retr): Report pop errors separately, since caller now does
errno reporting.
(mbx_write, mbx_delimit_begin, mbx_delimit_end): Check < 0, not ==
EOF, as it's a bit faster and (in theory) pickier.