libpromises Str2Uid/Str2Gid functions ID check

The Str2Uid/Str2Gid functions in libpromises/conversion.c check only the first character of the provided string to see if it is a digit - if so the whole string is treated as an ID. This breaks handling of access/ownership promises for user/group names that start with a digit.

<pre><code>

1. groupadd -g 3000 3p-contractor
2. useradd -g 3p-contractor -u 3000 3p-contractor
3. touch /tmp/testfile8
4. chown 3p-contractor:3p-contractor /tmp/testfile8
5. ll /tmp/testfile8
   rw-rr-. 1 3p-contractor 3p-contractor 0 Jun 17 00:01 /tmp/testfile8
6. cat test.cf
   body common control {
   bundlesequence => { "go" }

   ;
   }

bundle agent go {
files:
"/tmp/testfile8"
perms => mog("0644", "3p-contractor", "3p-contractor"),
create => "true";
}

body perms mog(mode, user, group) {
owners =>

{ "$(user)" }

;
groups =>

{ "$(group)" }

;
mode => "$(mode)";
}

1. cf-agent -KIf ./test.cf
   2014-06-17T00:01:55+0200 info: /go/files/'/tmp/testfile8': Owner of '/tmp/testfile8' was 3000, setting to 3
   2014-06-17T00:01:55+0200 info: /go/files/'/tmp/testfile8': Group of '/tmp/testfile8' was 3000, setting to 3
2. ll /tmp/testfile8
   rw-rr-. 1 adm sys 0 Jun 17 00:01 /tmp/testfile8
3. grep ':x:3:' /etc/passwd /etc/group
   /etc/passwd:adm:x:3:4:adm:/var/adm:/sbin/nologin
   /etc/group:sys:x:3:bin,adm
   </pre></code>

The workaround is to specify UID/GIDs rather than user/group names.

See https://github.com/cfengine/core/pull/1756