Update macros.

Use TLS_MAX_SIGALGCNT for the maximum number of entries in the
signature algorithms array.

Use TLS_MAX_SIGSTRING_LEN for the maxiumum length of each signature
component instead of a magic number.

Reviewed-by: Richard Levitte <levitte@openssl.org>
Reviewed-by: Matt Caswell <matt@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/2301)