Introduce osmo_identifier_valid() function to check validity of identifier

We define the notion of an 'osmocom identifier' which is basically a
7-bit US-ASCII without any special characters beyond "-_:@".  We
introduce a function to verify if an identifier consists only of the
permitted characters.

Change-Id: I96a8d345c5a69238a12d040f39b70c485a5c421c
diff --git a/include/osmocom/core/utils.h b/include/osmocom/core/utils.h
index 4b083f6..c5cc138 100644
--- a/include/osmocom/core/utils.h
+++ b/include/osmocom/core/utils.h
@@ -22,6 +22,7 @@
 /*! Number of bytes necessary to store given BITS */
 #define OSMO_BYTES_FOR_BITS(BITS) ((BITS + 8 - 1) / 8)
 
+#include <stdbool.h>
 #include <stdint.h>
 #include <stdio.h>
 
@@ -112,4 +113,6 @@
 bool osmo_is_hexstr(const char *str, int min_digits, int max_digits,
 		    bool require_even);
 
+bool osmo_identifier_valid(const char *str);
+
 /*! @} */