libxbps: change the epoch char to ':' to differentiate the starting char in pkgname.

Added xbps_get_pkg_epoch(), to get the epoch version string.

include/xbps_api.h

@@ -1124,6 +1124,16 @@ char *xbps_get_pkg_name(const char *pkg);
  */
 char *xbps_get_pkgpattern_name(const char *pattern);
 
+/**
+ * Gets the package epoch version in a package string, i.e <b>foo-2.0:epoch</b>.
+ *
+ * @param[in] pkg Package string.
+ * 
+ * @return A string with the epoch version string, NULL if it couldn't find
+ * the epoch component.
+ */
+const char *xbps_get_pkg_epoch(const char *pkg);
+
 /**
  * Gets the package version in a package string, i.e <b>foo-2.0</b>.
  *