Allow FIFOs to be used as config files

There are some config options which have no equivalent command line
option. Some may want to set these options dynamically or on a one-off
basis where a static config file is not ideal, and one very easy way to
do that is using process substitution:

    linkchecker --config <(printf '%s\n' '[filtering]' 'ignorewarnings=http-redirected') ...

This, however, does not work in the current code because these are
typically implemented as FIFOs, which don't pass the `os.path.isfile`
check:

    WARNING linkcheck.cmdline 2023-10-31 00:12:09,678 MainThread Unreadable config file: '/dev/fd/63'

Allow reading FIFOs as config input so that this is possible.
`fileutil.is_readable` also now doesn't check if the path leads to a
regular file: this is only used as part of cookie and config file input,
and in both cases that's not really relevant.

linkcheck/command/linkchecker.py

@@ -135,7 +135,7 @@ def linkchecker():
         files = []
         if options.configfile:
             path = configuration.normpath(options.configfile)
-            if os.path.isfile(path):
+            if fileutil.is_valid_config_source(path):
                 files.append(path)
             else:
                 log.warn(

linkcheck/configuration/__init__.py

@@ -222,7 +222,7 @@ class Configuration(dict):
         # filter invalid files
         filtered_cfiles = []
         for cfile in cfiles:
-            if not os.path.isfile(cfile):
+            if not fileutil.is_valid_config_source(cfile):
                 log.warn(LOG_CHECK, _("Configuration file %r does not exist."), cfile)
             elif not fileutil.is_readable(cfile):
                 log.warn(LOG_CHECK, _("Configuration file %r is not readable."), cfile)

linkcheck/fileutil.py

@@ -89,8 +89,8 @@ def is_tty(fp):
 
 @lru_cache(128)
 def is_readable(filename):
-    """Check if file is a regular file and is readable."""
-    return os.path.isfile(filename) and os.access(filename, os.R_OK)
+    """Check if file is readable."""
+    return os.access(filename, os.R_OK)
 
 
 def is_accessable_by_others(filename):
@@ -103,3 +103,8 @@ def is_writable_by_others(filename):
     """Check if file or directory is world writable."""
     mode = os.stat(filename)[stat.ST_MODE]
     return mode & stat.S_IWOTH
+
+
+def is_valid_config_source(filename):
+    """Check if the file is a valid config file."""
+    return os.path.isfile(filename) or stat.S_ISFIFO(os.stat(filename).st_mode)