Add documentation for X509 time functions

=head1 SYNOPSIS

 X509_cmp_time(const ASN1_TIME *asn1_time, time_t *cmp_time);

 int X509_cmp_time(const ASN1_TIME *asn1_time, time_t *in_tm);

 int X509_cmp_current_time(const ASN1_TIME *asn1_time);

 ASN1_TIME *X509_time_adj(ASN1_TIME *asn1_time, long offset_sec, time_t *in_tm);

 ASN1_TIME *X509_time_adj_ex(ASN1_TIME *asn1_time, int offset_day, long

                             offset_sec, time_t *in_tm);

=head1 DESCRIPTION

X509_cmp_time() compares the ASN1_TIME in B<asn1_time> with the time in

<cmp_time>.

X509_cmp_time() compares the ASN1_TIME in B<asn1_time> with the time

in <cmp_time>. X509_cmp_current_time() compares the ASN1_TIME in

B<asn1_time> with the current time, expressed as time_t. B<asn1_time>

must satisfy the ASN1_TIME format mandated by RFC 5280, i.e., its

format must be either YYMMDDHHMMSSZ or YYYYMMDDHHMMSSZ.

B<asn1_time> must satisfy the ASN1_TIME format mandated by RFC 5280, i.e.,

its format must be either YYMMDDHHMMSSZ or YYYYMMDDHHMMSSZ.

X509_time_adj_ex() sets the ASN1_TIME structure B<asn1_time> to the time

B<offset_day> and B<offset_sec> after B<in_tm>.

If B<cmp_time> is NULL the current time is used.

X509_time_adj() sets the ASN1_TIME structure B<asn1_time> to the time

B<offset_sec> after B<in_tm>. This method can only handle second

offsets up to the capacity of long, so the newer X509_time_adj_ex()

API should be preferred.

In both methods, if B<asn1_time> is NULL, a new ASN1_TIME structure

is allocated and returned.

In all methods, if B<in_tm> is NULL, the current time, expressed as

time_t, is used.

=head1 BUGS

Unlike many standard comparison functions, X509_cmp_time returns 0 on error.

Unlike many standard comparison functions, X509_cmp_time() and

X509_cmp_current_time() return 0 on error.

=head1 RETURN VALUES

X509_cmp_time() returns -1 if B<asn1_time> is earlier than, or equal to,

B<cmp_time>, and 1 otherwise. It returns 0 on error.

X509_cmp_time() and X509_cmp_current_time() return -1 if B<asn1_time>

is earlier than, or equal to, B<cmp_time> (resp. current time), and 1

otherwise. These methods return 0 on error.

X509_time_ad() and X509_time_adj_ex() return a pointer to the updated

ASN1_TIME structure, and NULL on error.

=head1 COPYRIGHT