go.sys/unix: use syscall.Errno for errors

If we use a local type, it won't compare properly with errors from
the rest of the standard library. Errors are the one type from syscall
that propagates through the system, so it's important to have only
one type for them.

Ditto for syscall.Signal.

LGTM=dave
R=rsc, dave
CC=golang-codereviews
https://golang.org/cl/123490043

unix/mkerrors.sh

@@ -42,6 +42,7 @@ ccflags="$@"
 	echo "${!indirect} $includes"
 	echo '*/'
 	echo 'import "C"'
+	echo 'import "syscall"'
 	echo
 	echo 'const ('
 
@@ -147,13 +148,13 @@ cat _error.out | grep -vf _error.grep | grep -vf _signal.grep
 echo
 echo '// Errors'
 echo 'const ('
-cat _error.out | grep -f _error.grep | sed 's/=\(.*\)/= Errno(\1)/'
+cat _error.out | grep -f _error.grep | sed 's/=\(.*\)/= syscall.Errno(\1)/'
 echo ')'
 
 echo
 echo '// Signals'
 echo 'const ('
-cat _error.out | grep -f _signal.grep | sed 's/=\(.*\)/= Signal(\1)/'
+cat _error.out | grep -f _signal.grep | sed 's/=\(.*\)/= syscall.Signal(\1)/'
 echo ')'
 
 # Run C program to print error and syscall strings.