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/cryptsetup/cryptsetup.c

@@ -127,7 +127,7 @@ static int parse_one_option(const char *option) {
                 opt_type = CRYPT_PLAIN;
         else if (startswith(option, "timeout=")) {
 
-                if (parse_usec(option+8, &opt_timeout) < 0) {
+                if (parse_sec(option+8, &opt_timeout) < 0) {
                         log_error("timeout= parse failure, ignoring.");
                         return 0;
                 }