Some cleanups to apps/ca.c

Few code format fixup
Fix limit computation; was too strict by 2 bytes.
Simplify computation of buffer limits
Checking is strictly same as sizeof(".pem") == 5
Simplify loop of code for certificate filename creation
Fix MAX_PATH usage

Reviewed-by: Paul Dale <paul.dale@oracle.com>
Reviewed-by: Rich Salz <rsalz@openssl.org>
(Merged from https://github.com/openssl/openssl/pull/1936)