Remedy gcc 7.4.0 warnings (#187)

* Use logical not for booleans

Fixes a gcc 7.4.0 complaint:

sshfs.c:1743:28: warning: ‘~’ on a boolean expression [-Wbool-operation]
sshfs.c:1743:28: note: did you mean to use logical not?

* strdup argument to keep it alive after function return

Fixes gcc 7.4.0 complaint:

sshfs.c: In function ‘sshfs_opt_proc’:
sshfs.c:3488:50: warning: assignment discards ‘const’ qualifier from pointer target type [-Wdiscarded-qualifiers]

sshfs.c

@@ -1745,7 +1745,7 @@ static void *sshfs_init(struct fuse_conn_info *conn,
  sshfs.sync_read = 1;
 
  // These workarounds require the "path" argument.
- cfg->nullpath_ok = ~(sshfs.truncate_workaround || sshfs.fstat_workaround);
+ cfg->nullpath_ok = !(sshfs.truncate_workaround || sshfs.fstat_workaround);
 
  // Lookup of . and .. is supported
  conn->capable |= FUSE_CAP_EXPORT_SUPPORT;
@@ -3494,7 +3494,7 @@ static int sshfs_opt_proc(void *data, const char *arg, int key,
  * mountpoint to later be completely
  * unprivileged with libfuse >= 3.3.0.
  */
- sshfs.mountpoint = arg;
+ sshfs.mountpoint = strdup(arg);
  } else {
  sshfs.mountpoint = realpath(arg, NULL);
  }