writeout: add %time{}

Output the current UTC time using strftime format. %f is an extra curl
specific flag to output the microsecond fraction of the current second.

Verified by test 1981

tests/data/Makefile.am

@@ -238,7 +238,7 @@ test1933 test1934 test1935 test1936 test1937 test1938 test1939 test1940 \
 test1941 test1942 test1943 test1944 test1945 test1946 test1947 test1948 \
 test1955 test1956 test1957 test1958 test1959 test1960 test1964 \
 test1970 test1971 test1972 test1973 test1974 test1975 test1976 test1977 \
-test1978 test1979 test1980 \
+test1978 test1979 test1980 test1981 \
 \
 test2000 test2001 test2002 test2003 test2004 test2005 \
 \

tests/data/test1981

@@ -0,0 +1,62 @@
+<testcase>
+<info>
+<keywords>
+HTTP
+HTTP GET
+</keywords>
+</info>
+
+#
+# Server-side
+<reply>
+<data crlf="yes" nocheck="yes">
+HTTP/1.1 200 OK
+Date: Tue, 09 Nov 2010 14:49:00 GMT
+Server: test-server/fake
+Last-Modified: Tue, 13 Jun 2000 12:10:00 GMT
+ETag: "21025-dc7-39462498"
+Accept-Ranges: bytes
+Content-Length: 6
+Connection: close
+Content-Type: text/html
+Funny-head: yesyes
+
+-foo-
+</data>
+</reply>
+
+#
+# Client-side
+<client>
+<server>
+http
+</server>
+<name>
+%time output with --write-out
+</name>
+<features>
+Debug
+</features>
+<setenv>
+CURL_TIME=1754037100
+</setenv>
+<command>
+http://%HOSTIP:%HTTPPORT/%TESTNUMBER --write-out='Time: %time{%d/%b/%Y %H:%M:%S.%f %z}\n' -s -o %LOGDIR/dump
+</command>
+</client>
+
+#
+# Verify data after the test has been "shot"
+<verify>
+<protocol crlf="yes">
+GET /%TESTNUMBER HTTP/1.1
+Host: %HOSTIP:%HTTPPORT
+User-Agent: curl/%VERSION
+Accept: */*
+
+</protocol>
+<stdout mode="text">
+Time: 01/Aug/2025 08:31:40.037100 +0000
+</stdout>
+</verify>
+</testcase>