feat(std/fs): add support to parse home directory from /etc/passwd

[jobansd]

Implemented the TODO in get_app_data_dir.zig that suggests parsing the home directory from /etc/passwd.

Changes

• Added getHomeDirFromPasswd() function, which implements the parsing logic.
• Changed the return type of getAppDataDir() from GetAppDataDirError![]u8 to ![]u8 to encompass errors from fs.openFileAbsolute() and reader.read()
• GetAppDataDirError retained in return type, updated the getHomeDirFromPasswd() function call to catch and return error.AppDataDirUnavailable instead (suggested by @squeek502).
• Added declaration for getuid() to c.zig.
• Added wrapper function for getuid() to posix.zig.
• Changed std.os.linux.getuid() to std.posix.getuid() in get_app_data_dir.zig.

[squeek502]

IMO the GetAppDataDirError error set should be retained, and the getHomeDirFromPasswd calls should be changed to

getHomeDirFromPasswd() catch return error.AppDataDirUnavailable

so something like

const home_dir = posix.getenv("HOME") orelse (getHomeDirFromPasswd() catch return error.AppDataDirUnavailable);

or

const home_dir = posix.getenv("HOME") orelse blk: {
    break :blk getHomeDirFromPasswd() catch return error.AppDataDirUnavailable;
};

[jobansd]

@squeek502, thanks for the suggestion, the change has been pushed.