Disable IPsec debugging code by default when IPSEC_DEBUG kernel option
is not specified.

Due to the long call chain IPsec code can produce the kernel stack
exhaustion on the i386 architecture. The debugging code usually is not
used, but it requires a lot of stack space to keep buffers for strings
formatting. This patch conditionally defines macros to disable building
of IPsec debugging code.

IPsec currently has two sysctl variables to configure debug output:

• net.key.debug variable is used to enable debug output for PF_KEY protocol. Such debug messages are produced by KEYDBG() macro and usually they can be interesting for developers.
• net.inet.ipsec.debug variable is used to enable debug output for DPRINTF() macro and ipseclog() function. DPRINTF() macro usually is used for development debugging. ipseclog() function is used for debugging by administrator.

The patch disables KEYDBG() and DPRINTF() macros, and formatting buffers
declarations when IPSEC_DEBUG is not present in kernel config. This reduces
stack requirement for up to several hundreds of bytes.
The net.inet.ipsec.debug variable still can be used to enable ipseclog()
messages by administrator.

PR: 219476
Reported by: eugen
No objection from: network
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D10869