Add FreeBSD support

[vkrivopalov]

Port MTProxy to FreeBSD by addressing Linux-specific dependencies:

Build system (Makefile):

• Add FreeBSD detection and platform-specific CFLAGS/LDFLAGS
• Link against libepoll-shim for epoll compatibility layer
• Link against libexecinfo for backtrace support
• Remove -lrt and -rdynamic (not needed on FreeBSD)

Signal handling:

• Add missing signal.h include in mtproto-proxy.c
• Adapt SIGRTMAX handling for FreeBSD (uses different signal range)
• Wrap SIGPOLL usage (not available on FreeBSD)

Threading and synchronization:

• Replace Linux futex syscalls with FreeBSD _umtx_op()
• Replace SYS_gettid with thr_self() for thread identification
• Replace glibc-specific drand48_r/srand48_r with mutex-protected standard drand48/srand48 functions
• Wrap struct drand48_data with #ifdef linux

Socket options:

• Wrap TCP_WINDOW_CLAMP (Linux-specific)
• Wrap TCP_QUICKACK (Linux-specific)
• Wrap IP_RECVERR/SOL_IP (Linux-specific)

Header compatibility:

• Disable _XOPEN_SOURCE on FreeBSD (conflicts with IPv6 definitions)
• Add sys/socket.h before netinet/in.h for struct sockaddr_in6
• Add socket headers to resolver.c
• Wrap sys/io.h include (Linux-specific)
• Undef close macro from libepoll-shim to avoid struct member conflict

Compiler compatibility:

• Add clang-compatible intrinsic macros for SIMD operations in crc32.c (maps GCC _builtin_ia32* to standard Intel intrinsics)

Requires: libepoll-shim package (pkg install libepoll-shim)

Use 'gmake' (pkg install gmake) to stay compatible with GNU-style Makefile.

Closes: #16 #331