Use standard Fortran functions for getting command-line arguments

[billsacks]

Test suite: Manual testing: compared before and after output from printing args with these diffs:

diff --git a/tools/mapping/gen_domain_files/src/gen_domain.F90 b/tools/mapping/gen_domain_files/src/gen_domain.F90
index f0b97f20e..289fbce6b 100644
--- a/tools/mapping/gen_domain_files/src/gen_domain.F90
+++ b/tools/mapping/gen_domain_files/src/gen_domain.F90
@@ -118,6 +118,15 @@ program fmain
     end select
   end do
 
+  print *, 'fmap = ', trim(fmap)
+  print *, 'fn1_out = ', trim(fn1_out)
+  print *, 'fn2_out = ', trim(fn2_out)
+  print *, 'set_fv_pole_yc = ', set_fv_pole_yc
+  print *, 'fminval = ', fminval
+  print *, 'fmaxval = ', fmaxval
+  print *, 'usercomment = ', trim(usercomment)
+  print *, 'set_omask = ', set_omask
+
   if (fmap == 'null' .or. fn1_out == 'null' .or. fn2_out== 'null') then
     call usage_exit ('Must specify all the following arguments')
   end if

Test baseline: n/a
Test namelist changes: none
Test status: bit for bit

Fixes #4455

User interface changes?: N

Update gh-pages html (Y/N)?: N

[jasonb5]

LGTM