Change all whoami calls for $USER variable

This avoid spawning additional processes, as the $USER variable will
**always** contain the current logged-in user.

themes/adben.zsh-theme

@@ -89,9 +89,8 @@ function precmd {
 
 # Context: user@directory or just directory
 prompt_context () {
-    local user=`whoami`
-    if [[ "$user" != "$DEFAULT_USER" || -n "$SSH_CLIENT" ]]; then
-        echo -n "${PR_RESET}${PR_RED}$user@%m${PR_RESET}${PR_BRIGHT_YELLOW}%~%<<${PR_RESET}"
+    if [[ "$USER" != "$DEFAULT_USER" || -n "$SSH_CLIENT" ]]; then
+        echo -n "${PR_RESET}${PR_RED}$USER@%m${PR_RESET}${PR_BRIGHT_YELLOW}%~%<<${PR_RESET}"
     else
         echo -n "${PR_RESET}${PR_BRIGHT_YELLOW}%~%<<${PR_RESET}"
     fi

themes/agnoster.zsh-theme

@@ -60,10 +60,8 @@ prompt_end() {
 
 # Context: user@hostname (who am I and where am I)
 prompt_context() {
-  local user=`whoami`
-
-  if [[ "$user" != "$DEFAULT_USER" || -n "$SSH_CLIENT" ]]; then
-    prompt_segment black default "%(!.%{%F{yellow}%}.)$user@%m"
+  if [[ "$USER" != "$DEFAULT_USER" || -n "$SSH_CLIENT" ]]; then
+    prompt_segment black default "%(!.%{%F{yellow}%}.)$USER@%m"
   fi
 }
 

themes/flazz.zsh-theme

@@ -1,4 +1,4 @@
-if [ "$(whoami)" = "root" ]
+if [ "$USER" = "root" ]
 then CARETCOLOR="red"
 else CARETCOLOR="blue"
 fi

themes/obraun.zsh-theme

@@ -1,4 +1,4 @@
-if [ "$(whoami)" = "root" ]; then CARETCOLOR="red"; else CARETCOLOR="blue"; fi
+if [ "$USER" = "root" ]; then CARETCOLOR="red"; else CARETCOLOR="blue"; fi
 
 local return_code="%(?..%{$fg[red]%}%? ↵%{$reset_color%})"
 

themes/re5et.zsh-theme

@@ -1,4 +1,4 @@
-if [ "$(whoami)" = "root" ]; then CARETCOLOR="red"; else CARETCOLOR="magenta"; fi
+if [ "$USER" = "root" ]; then CARETCOLOR="red"; else CARETCOLOR="magenta"; fi
 
 local return_code="%(?..%{$fg_bold[red]%}:( %?%{$reset_color%})"
 

themes/sunrise.zsh-theme

@@ -10,7 +10,7 @@ Y=$fg_no_bold[yellow]
 B=$fg_no_bold[blue]
 RESET=$reset_color
 
-if [ "$(whoami)" = "root" ]; then
+if [ "$USER" = "root" ]; then
     PROMPTCOLOR="%{$R%}" PREFIX="-!-";
 else
     PROMPTCOLOR="" PREFIX="---";

themes/ys.zsh-theme

@@ -33,7 +33,7 @@ ${git_info} \
 %{$fg[white]%}[%*]
 %{$terminfo[bold]$fg[red]%}$ %{$reset_color%}"
 
-if [[ "$(whoami)" == "root" ]]; then
+if [[ "$USER" == "root" ]]; then
 PROMPT="
 %{$terminfo[bold]$fg[blue]%}#%{$reset_color%} \
 %{$bg[yellow]%}%{$fg[cyan]%}%n%{$reset_color%} \