ulimit: Change default to access soft limits (xonsh#3990)

* ulimit: Change default to access soft limits

It makes more sense to manipulate "soft" limits by default, because
those are the limits intended to be user-adjustable. This is also
what bash does. Originally, I made ulimit to default to "hard" limits,
because I was for some reason convinced that it's what bash does.

* Changelog entry

news/ulimit-default-soft.rst

@@ -0,0 +1,23 @@
+**Added:**
+
+* <news item>
+
+**Changed:**
+
+* ``ulimit`` builtin now operates on "soft" limits by default.
+
+**Deprecated:**
+
+* <news item>
+
+**Removed:**
+
+* <news item>
+
+**Fixed:**
+
+* <news item>
+
+**Security:**
+
+* <news item>

xonsh/xoreutils/ulimit.py

@@ -195,8 +195,8 @@ def _ul_parse_args(args, stderr):
         long_opts[_UL_RES[k][1]] = k
 
     actions = []
-    # mimic bash and default to 'hard' limits
-    res_type = _UL_HARD
+    # mimic bash and default to 'soft' limits
+    res_type = _UL_SOFT
     for arg in args:
         if arg in long_opts:
             opt = long_opts[arg]