Simplify build tags for restic mount

This command can only be built on Darwin, FreeBSD and Linux
(and if we upgrade bazil.org/fuse, only FreeBSD and Linux:
https://github.com/bazil/fuse/issues/224).

Listing the few supported operating systems explicitly here makes
porting restic to new platforms easier.

cmd/restic/cmd_mount.go

@@ -1,7 +1,4 @@
-// +build !netbsd
-// +build !openbsd
-// +build !solaris
-// +build !windows
+// +build darwin freebsd linux
 
 package main
 

internal/fuse/blob_size_cache.go

@@ -1,7 +1,4 @@
-// +build !netbsd
-// +build !openbsd
-// +build !solaris
-// +build !windows
+// +build darwin freebsd linux
 
 package fuse
 

internal/fuse/dir.go

@@ -1,7 +1,4 @@
-// +build !netbsd
-// +build !openbsd
-// +build !solaris
-// +build !windows
+// +build darwin freebsd linux
 
 package fuse
 

internal/fuse/file.go

@@ -1,7 +1,4 @@
-// +build !netbsd
-// +build !openbsd
-// +build !solaris
-// +build !windows
+// +build darwin freebsd linux
 
 package fuse
 

internal/fuse/fuse_test.go

@@ -1,7 +1,4 @@
-// +build !netbsd
-// +build !openbsd
-// +build !solaris
-// +build !windows
+// +build darwin freebsd linux
 
 package fuse
 

internal/fuse/link.go

@@ -1,7 +1,4 @@
-// +build !netbsd
-// +build !openbsd
-// +build !solaris
-// +build !windows
+// +build darwin freebsd linux
 
 package fuse
 

internal/fuse/meta_dir.go

@@ -1,7 +1,4 @@
-// +build !netbsd
-// +build !openbsd
-// +build !solaris
-// +build !windows
+// +build darwin freebsd linux
 
 package fuse
 

internal/fuse/other.go

@@ -1,7 +1,4 @@
-// +build !netbsd
-// +build !openbsd
-// +build !solaris
-// +build !windows
+// +build darwin freebsd linux
 
 package fuse
 

internal/fuse/root.go

@@ -1,7 +1,4 @@
-// +build !netbsd
-// +build !openbsd
-// +build !solaris
-// +build !windows
+// +build darwin freebsd linux
 
 package fuse
 

internal/fuse/snapshots_dir.go

@@ -1,7 +1,4 @@
-// +build !netbsd
-// +build !openbsd
-// +build !solaris
-// +build !windows
+// +build darwin freebsd linux
 
 package fuse