implement -nnn style flags in argparse

This implements support for numeric flags without an associated short or
long flag name. This pattern is used by many commands. For example `head
-3 /a/file` to emit the first three lines of the file.

Fixes #4214

tests/string.err

@@ -5,7 +5,7 @@ string match: ^
 
 # string invalidarg
 string: Subcommand 'invalidarg' is not valid
-Standard input (line 258): 
+Standard input (line 251): 
 string invalidarg; and echo "unexpected exit 0" >&2
 ^
 
@@ -29,6 +29,6 @@ string repeat: Expected argument
 
 # string repeat -l fakearg 2>&1
 string repeat: Unknown option '-l'
-Standard input (line 359): 
+Standard input (line 352): 
 string repeat -l fakearg
 ^