Support qmail if the QMAIL macro is defined. This entails exiting 111

Support qmail if the QMAIL macro is defined.  This entails exiting 111
on failure, not having to worry about dumbass /^From /, and handling
RPLINE and DTLINE.

Say EX_TEMPFAIL instead of MDELIVER_EXITCODE.

s/TMPNAMLEN/LEN_FILENAME/; it's not just used for the tmp file.

Random cleanup:
- drop some redundant comments
- s/statbuf/st/
- s/STDIN_FILENO/0/
- cleanup error messages
- other style nits

Drop <err.h> and implement custom err and errx, which doesn't need to
be passed an exit code.

Implement and use full_sleep to really sleep the specified time.

(get_filename): Don't break when gethostname truncates.

(strip_from): Don't even compile this for QMAIL.

(deliver): Take a new parameter, the subdirectory to deliver to,
instead of "new".  #ifdef QMAIL, write RPLINE and DTLINE to tmp file
before copying stdin.

(deliver): Also had a huge problem: it was using the logic i came up
with for minc, to open an empty file for the target and then rename
onto that.  That's fine for mh, which already loses mail this way
(once i switch refile to be like minc this will be fine), but since
other Maildir MDAs follow the official algorithm, this could clobber
mail.  Switch to the official algorithm.

(run_processor): Use better error message format.

(main): If .mdeliver-processor exists, pass deliver "mdb" as the
subdirectory and then call run_processor, else pass "new".