util: Avoid memory allocations for formatting paths

Avoid memory allocations to construct the path for files in the
procfs. The procfs paths are way shorter than the PATH_MAX so we
can use snprintf on a string located on the stack. This shows up
as a win on x86 using the benchmark program below.

$ make libsystemd-shared.la; gcc -O2 -Isrc/systemd/ -Isrc/ \
	-o simple-perf-test simple-perf-test.c \
	.libs/libsystemd-shared.a  -lrt

 #include "shared/util.h"
void test_once(void) {
	pid_t pid = getpid();
	char *tmp = NULL;

	get_process_comm(pid, &tmp);
	free(tmp);
	tmp = NULL;
	get_process_cmdline(pid, 0, 1, &tmp);
	free(tmp);
	is_kernel_thread(pid);
	tmp = NULL;
	get_process_exe(pid, &tmp);
	free(tmp);
}

int main(int argc, char **argv)
{
	int i;
	for (i = 0; i < 50000; ++i)
		test_once();
}

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