util: rename parse_usec() to parse_sec() sinds the default unit is seconds

Internally we store all time values in usec_t, however parse_usec()
actually was used mostly to parse values in seconds (unless explicit
units were specified to define a different unit). Hence, be clear about
this and name the function about what we pass into it, not what we get
out of it.

src/tmpfiles/tmpfiles.c

@@ -1171,7 +1171,7 @@ static int parse_line(const char *fname, unsigned line, const char *buffer) {
                         a++;
                 }
 
-                if (parse_usec(a, &i->age) < 0) {
+                if (parse_sec(a, &i->age) < 0) {
                         log_error("[%s:%u] Invalid age '%s'.", fname, line, age);
                         return -EBADMSG;
                 }