Allow to override UserAgent

Adding on command line the user agent add a duplicate header:

```
./w3m -header "User-Agent: Mozilla" http://localhost:9999

GET / HTTP/1.0
User-Agent: w3m/0.5.3+git20190105
Accept: text/html, text/*;q=0.5, image/*, application/*, message/*, x-scheme-handler/*, audio/*, video/*, inode/*
Accept-Encoding: gzip, compress, bzip, bzip2, deflate
Accept-Language: en;q=1.0
Host: localhost:9999
Pragma: no-cache
Cache-control: no-cache
User-Agent: Mozilla
```

As a result most server will take the first given; the default
w3m_version or the one defined on config `user_agent`

With this patch we can now override `User-Agent` from command line

1 files changed, 2 insertions, 0 deletions

diff --git a/main.c b/main.c
index 43e181c..7bb058e 100644
--- a/main.c
+++ b/main.c
@@ -375,6 +375,8 @@ make_optional_header_string(char *s)
     Strcopy_charp_n(hs, s, p - s);
     if (!Strcasecmp_charp(hs, "content-type"))
 	override_content_type = TRUE;
+    if (!Strcasecmp_charp(hs, "user-agent"))
+	override_user_agent = TRUE;
     Strcat_charp(hs, ": ");
     if (*(++p)) {		/* not null header */
 	SKIP_BLANKS(p);		/* skip white spaces */