Add note about servers with weak DH group parameters
Include <unistd.h> in src/util.h

On FreeBSD 11-CURRENT this fixes the compilation failure:

In file included from ./src/proxy-bio.h:16,
                 from src/proxy-bio.c:38:
./src/util.h:46: error: expected '=', ',', ';', 'asm' or '__attribute__' before 'wait_with_timeout'
./src/util.h:50: warning: 'struct timeval' declared inside parameter list
./src/util.h:50: warning: its scope is only this definition or declaration, which is probably not what you want
./src/util.h:51: warning: 'struct timeval' declared inside parameter list
./src/util.h:57: warning: 'struct timeval' declared inside parameter list
*** Error code 1
Include <sys/stat.h> in src/util.c

On FreeBSD 11-CURRENT this fixes the compilation failure:

src/util.c: In function 'file_write':
src/util.c:256: error: 'S_IRUSR' undeclared (first use in this function)
src/util.c:256: error: (Each undeclared identifier is reported only once
src/util.c:256: error: for each function it appears in.)
src/util.c:256: error: 'S_IWUSR' undeclared (first use in this function)
*** Error code 1