strtoul() is required to parse negative numbers as their
twos-complement positive counterpart.  But sometimes we want
to reject negative numbers.  Add new functions to do this.
The 'p' suffix is a mnemonic for 'positive' (technically it
also parses 0, but 'non-negative' doesn't lend itself to a
nice one-letter suffix).

* src/util/virstring.h (virStrToLong_uip, virStrToLong_ulp)
(virStrToLong_ullp): New prototypes.
* src/util/virstring.c (virStrToLong_uip, virStrToLong_ulp)
(virStrToLong_ullp): New functions.
* src/libvirt_private.syms (virstring.h): Export them.
* tests/virstringtest.c (testStringToLong): Test them.
Signed-off-by: NEric Blake <eblake@redhat.com>