- Loop over USES twice, once to define all *_ARGS variables and once to
  include Uses/*.mk.  This allows all Uses/*.mk to examine arguments given
  to other USES entries.
- Always define *_ARGS (possibly empty) and replace commas with spaces.

Similar for _USES_POST.

Adjust all Uses/*.mk:
- defined(u_ARGS) becomes !empty(u_ARGS)
- Eliminate helper variables like _*_ARGS=${*_ARGS:C/,/ /g}
- Some Uses/*.mk used ":" as argument separator instead of ",", but no port
  used this form
- Uses/cran.mk: remove unused variable VALID_ARGS and USES+=fortran which
  has no effect
- Uses/twisted.mk: simplify handling of the case where neither "build" nor
  "run" arguments have been specified

PR:		193931
Exp-run by:	antoine
Approved by:	portmgr (antoine)