util: buffer: Properly URLencode strings

According to rfc3986:

2.3.  Unreserved Characters

   Characters that are allowed in a URI but do not have a reserved
   purpose are called unreserved.  These include uppercase and lowercase
   letters, decimal digits, hyphen, period, underscore, and tilde.

      unreserved  = ALPHA / DIGIT / "-" / "." / "_" / "~"

   URIs that differ in the replacement of an unreserved character with
   its corresponding percent-encoded US-ASCII octet are equivalent: they
   identify the same resource.  However, URI comparison implementations
   do not always perform normalization prior to comparison (see Section
   6).  For consistency, percent-encoded octets in the ranges of ALPHA
   (%41-%5A and %61-%7A), DIGIT (%30-%39), hyphen (%2D), period (%2E),
   underscore (%5F), or tilde (%7E) should not be created by URI
   producers and, when found in a URI, should be decoded to their
   corresponding unreserved characters by URI normalizers.

Thus we must not include few other characters which don't match
c_isalpha to conform to the rules.

Signed-off-by: Peter Krempa <pkrempa@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>

tests/viruritest.c

@@ -184,7 +184,7 @@ mymain(void)
         { NULL, NULL, false },
     };
     TEST_FULL("spice://[3ffe::104]:5900/?tlsSubject=C=XX,L=Testtown,O=Test%20Company,CN=tester.test",
-              "spice://[3ffe::104]:5900/?tlsSubject=C%3dXX%2cL%3dTesttown%2cO%3dTest%20Company%2cCN%3dtester%2etest",
+              "spice://[3ffe::104]:5900/?tlsSubject=C%3dXX%2cL%3dTesttown%2cO%3dTest%20Company%2cCN%3dtester.test",
               "spice", "3ffe::104", 5900, "/", "tlsSubject=C=XX,L=Testtown,O=Test%20Company,CN=tester.test", NULL, NULL, spiceparams);
 
     virURIParam params1[] = {