Use availableParallelism, handle empty os.cpus()

When /proc is not available and os.cpus() returns an empty list,
callLimit() doesn't run any of the functions. Fix this and also update
tap which was broken in newer node versions

PR-URL: #11
Credit: @codedokode
Close: #11
Reviewed-by: @isaacs

EDIT(@isaacs): formatted commit message and comment for line length,
added coverage ignore for node version-specific workaround

index.js

@@ -1,4 +1,13 @@
-const defLimit = require('os').cpus().length
+const os = require('os')
+// availableParallelism available only since node v19, for older versions use
+// cpus() cpus() can return an empty list if /proc is not mounted, use 1 in
+// this case
+
+/* istanbul ignore next - version-specific workaround */
+const defLimit = 'availableParallelism' in os
+  ? os.availableParallelism()
+  : Math.max(1, os.cpus().length)
+
 const callLimit = (queue, limit = defLimit) => new Promise((res, rej) => {
   let active = 0
   let current = 0