user: compile path.c only once

Also merge bsd-user/path.c and linux-user/path.c.

Signed-off-by: Blue Swirl <blauwirbel@gmail.com>

bsd-user/qemu.h

@@ -137,8 +137,6 @@ abi_long do_openbsd_syscall(void *cpu_env, int num, abi_long arg1,
 void gemu_log(const char *fmt, ...) __attribute__((format(printf,1,2)));
 extern THREAD CPUState *thread_env;
 void cpu_loop(CPUState *env, enum BSDType bsd_type);
-void init_paths(const char *prefix);
-const char *path(const char *pathname);
 char *target_strerror(int err);
 int get_osversion(void);
 void fork_start(void);