fs(3lua): init man pages

Initialise man pages for the fs module.

man/fs/fs.workdir.3lua.scd

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+FS.WORKDIR(3lua) "3lua" "Callisto manual pages"
+
+# NAME
+*fs.workdir* - get or set the current working directory
+
+# SYNOPSIS
+*fs.workdir*([_dir_: *string*])
+
+# DESCRIPTION
+If _dir_ is not nil, *fs.workdir* sets the current working directory
+to the path _dir_.
+This means the starting point of all pathnames not beginning with a
+slash ('/') will be set to _dir_ until the working directory is
+changed again by a call to *fs.workdir* or chdir(2).
+
+If _dir_ is nil, *fs.workdir* returns the current working directory.
+
+If an error occurs,
+*fs.workdir* returns nil, an error message and
+a platform-dependent error code.
+There are many reasons *fs.workdir* can fail;
+see chdir(2) for possible error values
+when setting the current working directory,
+and getcwd(2) for possible error values
+when getting the current working directory.
+
+# EXAMPLES
+Get the current working directory:
+
+	local wd = fs.workdir()
+
+Set the current working directory to the user's home directory:
+
+	fs.workdir(environ["HOME"])
+
+# SEE ALSO
+callisto(3lua), fs(3lua), chdir(2), getcwd(2)