lib: IsNameValid(): disallow leading and trailing spaces

[pstorz]

Names with leading and trailing space are now illegal.
Also added a unit test that checks that IsNameValid() works
as expected.

[arogge]

Looks good. Maybe we also want to require that names to start with one of [A-Za-z0-9]?

[arogge]

If we added a second parameter const char* accept_additional that defaults to "" we could check this with strchr() in addition to accept so the caller could pass additional valid characters.

Then it should be possible to implement IsAclEntryValid() like this

bool IsAclEntryValid(const char* acl, char* msg) {
  if (acl[0] == '!') ++acl; // negated resource name/pattern, skip negation sign
  return IsNameValid(acl, msg, '.*')
}