(PIDSTR_DECL_AND_INIT): Don't assume pid_t fits in int.

(PIDSTR_DECL_AND_INIT): Don't assume pid_t fits in int.
(UT_ID) [!HAVE_STRUCT_XTMP_UT_ID]: Remove bogus comment,
as (sizeof "??") reliably returns 3.
(print_line): Guard against idle and pid being too long
(which is possible when printing headers).
(print_user): Allocate enough bytes for idlestr.  Use IDLESTR_LEN.
Avoid unnecessary cast of sizeof to int.
(make_id_equals_comment): Do not assume that UT_ID returns
a string; it might return a non-null-terminated array.
Use strncat instead.  It's not very often where strncat is
exactly what you want, but this is one of those rare cases.