add _nsec() syscall 53 for binary compatibility with labs distribution

the new syscall is added under the symbol _nsec() for
binary compatibility.

nsec() is still a library function reading /dev/bintime.

sys/lib/acid/syscall

@@ -130,6 +130,7 @@ syscalls = {
 	{50, {"pread",		"DADZ",		code(*syspread:list)}},
 	{51, {"pwrite",		"DTDZ",		code(*syspwrite:list)}},
 	{52, {"tsemacquire",	"AD",		code(*systsemacquire:list)}},
+	{53, {"_nsec",		"A",		code(*sys_nsec:list)}},
 };
 
 defn syscall() {

sys/lib/acid/truss

@@ -98,6 +98,7 @@ trusscalls = {
 		"pread",
 		"pwrite",
 		"tsemacquire",
+		"_nsec",
 	};
 
 trussapecalls = {