Clean up parsing of delimiters in -mdoc. First, remove the "dowarn"

Clean up parsing of delimiters in -mdoc.  First, remove the "dowarn"
variable from mandoc_getarg() so that it prints the warning every time.
Then, remove the warning from args_checkpunct().  This way, warnings
are being posted at the correct time.  This makes the flag argument to
mdoc_zargs() superfluous, so make it be zero when it's invoked.  Finally,
move the args() flags into mdoc_argv.c and make them enums.